The present invention relates to a measuring apparatus, and more precisely to a gravity meter. It uses a superconducting quantum interference device also known as SQUID.
There are three relevant types of prior art: gravimeters, SQUIDs and quantum gyroscopes.
1. Gravimeters
Gravimetry is an old art that has reached a level of precision and accuracy that few fields of science enjoy. There are different types of industries that are interested in gravimetry. A physicist might want to measure the variation of gravity with latitude, while a geophysicist will be interested in gravity in order to improve the current models of the interior of the Earth. The oil industry is also interested in gravity because a decrease in local gravity might indicate an oil field deep underground. This justifies the very large effort made into the development of equipment to obtain a more precise and accurate value of {right arrow over (g)} and {right arrow over (|g|)}.
There are basically three types of gravimeters: pendulum, spring and free-fall. The quest for ever more precise measurements has brought very many variations on the basic principle of an oscillating mass (pendulum), an elongated spring and free-falling objects. When measuring gravity, there are two types of measurements one is interested in: the absolute value of gravity at a given point and the variation of gravity with time at a given point. Different equipment will be used for these different measurements. Free-fall equipment clearly leads to an absolute value of gravity, while spring leads to a relative value of gravity. The-use of sprig equipment then requires the measurement of an absolute value of gravity at a given point that is used as a standard.
The modem free-fall equipment uses a laser beam directed on a retro-reflector that is in free-fall. The free-fall path is one arm of an interferometer, and one simply records the passage of interference fringes with time. From this information it is possible to extract the absolute value of g at the point where the free-fall occurred. Clearly, this value of g is the average value of g over the path of the free-fall Such devices have been designed for measurements on land (U.S. Pat. No. 3,727,462), in boreholes (U.S. Pat. No. 5,892,151) and in the water (U.S. Pat. No. 5,637,797).
The measurements of {right arrow over (f)} or {right arrow over (|g|)} have been so precise for the last 15 years, that time variation of {right arrow over (g)} have now been observed. The period of xe2x80x9coscillationxe2x80x9d varies from seconds to hours, and this phenomenon is still not very well understood. One possible explanation is that the continents oscillate due to the atmospheric pressure.
Superconductivity has also been used to measure gravity. An early attempt at using superconductivity in gravimetry is shown in U.S. Pat. No. 3,424,006 where a superconducting floating element is magnetically suspended in a superconducting ring. The upper face of this element is used as a mirror and constitutes one arm of an interferometer. If g changes with time, the suspended element will rise or fall in the superconducting ring, and this will lead to a shift in the interference fringes. One simply has to record the position of the fringes with the time and then deduce the stability of {right arrow over (g)} with time.
A more recent attempt at the use superconductivity in gravimetry-is-shown in U.S. Pat. No. 5,962,781. In this patent, a superconducting string is used as an antenna connected to driving solenoids in resonance. If {right arrow over (g)} changes with time, the position of the string will slightly change and the resonance will be lost. Since the system is in resonance, it is very sensitive to any variation of position or variation of {right arrow over (g.)}
Another recent use of superconductivity in gravimetry is shown in the design of GWR Instruments Inc., San Diego, Calif., USA, where a spinning superconducting sphere is suspended in a magnetic field. This superconducting magnetic field is very stable and acts essentially as a spring to support the bulk of the sphere. A second magnetic field is provided by a coil and the position of the sphere is provided by an electronic circuit where one of the components is the sphere. If {right arrow over (g)} changes with time, the sphere will slightly move in the magnetic fields and feed-back circuit changes the current in the coil in order to bring back the sphere to its original position. The change in current is produced by a change in voltage, and the voltage is simply recorded every few seconds or minutes depending on the user. This is a very good system to measure the stability of {right arrow over (g)} with time, but is not capable of measuring the absolute value of {right arrow over (g)} since the bulk of the weight of the sphere is supported by the superconducting magnetic field. This equipment provides the most sensitive data of the to variations of {right arrow over (g)} with time.
Most of the known devices have limitations and deficiencies. For example, the device that uses a spring system will suffer from loss of stiffness of the spring over time and is very sensitive to temperature changes. Also, most, if not all of them measure either {right arrow over (|g|)} or {right arrow over (g)}(t), but not both. The free-fall apparatus using a laser beam reaches a high precision of {right arrow over (|g|)} at a given point only after several measurements are combined in order to reduce the statistical error. For example, the device known as JILA-2 manufactured by Micro-G Instruments, Boulder, Colo., USA, requires about 2000 falls which will take 2-3 hours to measure. This equipment is not designed to monitor {right arrow over (g)}(t), primarily due to the wear and tear of the equipment. The same comment applies to the spinning superconducting sphere, since it is designed to monitor {right arrow over (g)}(t), but cannot get {right arrow over (|g|)}.
Another problem with the previous equipment is that of vibration: the free-fall device is not particularly sensitive to vibrations since its reference beam is suspended by a spring to cancel the vibrations through a retro-action electronic system. The GWR gravimeter, however, is very sensitive to vibrations in view of a mass suspended in a magnetic field. A third problem is the weight and portability of the equipment. Clearly, it is an asset to have the equipment that is light and can be easily carried to any point on Earth. The free-fall device (such as the JILA-2) is light and portable, but GWR equipment is very heavy (about 1 ton) and cannot be carried easily.
The purpose of the present invention is to solve these problems and to provide a single apparatus capable of measuring both {right arrow over (g)}(t) and {right arrow over (|g|)} very quickly and precisely. The apparatus of the present invention will allow to measure {right arrow over (|g|)} at a given instant, then monitor it for a certain period of time by measuring g(t), and then measure {right arrow over (|g|)} again to check for consistency. Another advantage of the present invention is the fact that it is practically immune to vibrations, thus opening a new venue of studies in geophysics, variations of {right arrow over (|g|)} during an earthquake and many other possibilities. The present invention is also unaffected by temperature, and can be made relatively light through to the use of a superconductors at high critical temperatures, thus greatly reducing the costs and bulkiness of the refrigerating equipment.
2. SQUIDS
Superconducting quantum interference devices (SQUIDs) are based on the quantization of the magnetic flux through a superconducting loop and on the Josephson effect; they are used mostly as very sensitive magnetic field detectors. In general, a SQUID is a loop of superconducting wire where one has built one or more Josephson junctions. When used as a magnetic field sensor, they are most often connected to a pick-up coil generally much larger than the SQUID itself. A slight variation of the ambient magnetic field will induce a current in the pick-up coil, which in turn induces a current the SQUID. This last current is quantized, which makes this a very sensitive device. SQUIDs have been used for several decades now as magnetic field sensors, and the technology is well known and is quite advanced For example, one can place 3 SQUIDs in a particular alignment in order to get {right arrow over (B)} in a single measurement, as shown in U.S. Pat. No. 5,786,690.
There is known another design of gravimeter that uses a SQUID comprising of a pair of masses that are part of an inductance circuit. This inductance itself is a part of an electrical circuit, and a SQUID is inductively connected to this circuit. If g varies, the masses will move relative to each other: this will change the inductance, then the current in the circuit, and finally, the current in the SQUID (see Paik, H. J., SQUID Applications to Geophysics, H. Weinstock and W. C. Overton eds., pp 3-12, Soc. of Exploration Geophysicists, Tulsa, Okla., 1981, Mapoles, E. A. ibid, pp 153-157).
3. Superconducting Gyroscopes
One of the first patents to use a phase in a gyroscope is shown in U.S. Pat. No. 3,657,927 to J. A. Tyson. The basic principle is that a wave moving in a medium will undergo a phase shift whose magnitude will depend on whether the wave in going in the direction of motion of the medium or whether it is going against the motion of the medium. This principle has been known for a long time and some designs were made at the beginning of the twentieth century to observe this phenomena with light. This principle is fundamental to the ring-laser gyroscope. The previously mentioned patent noted that in a superconductor, the Cooper pairs are coherent throughout the material, which is the basic principle of superconductivity. Since Cooper pairs can be thought of as a wave, it then follows that they will acquire a phase whether the supercurrent is going in the detection of motion of the superconductor or against its motion. Since the current in a SQUID depends greatly on the relative phases acquired along the paths that define the loop, one could expect that a SQUID be a very sensitive motion or rotation detector. This is indeed the basic idea of the above patent.
This principle has been used more recently in U.S. Pat. No. 5,058,431 showing that the critical variable for this effect to be observable is the area enclosed by the superconducting loop. The size of the induced phase shift is directly proportional to the area of the superconducting loop.
The apparatus of the present invention is basically a SQUID of a particular design which is shielded as well as possible from any magnetic field; since a magnetic field is highly undesirable, there are no pick-up coils connected to the SQUID of the present invention. The SQUID can be of macroscopic dimensions, but preferably it should be of microscopic dimensions. It can be fixed to a substrate or directly etched in a superconducting matrix. It is covered completely with a superconducting material leaving only the connectors uncovered. The whole system must be immersed in a cold liquid in the container in order to reach superconductivity. This container itself is mounted on a tiltable system that would allow the user to tilt the container at will. In order to get a measurement of the absolute value of {right arrow over (g,)}one simply tilts the container. This tilt will produce a phase shift the Cooper pairs of the SQUID""s loop, and this phase shit, which depends on {right arrow over (g,)}will produce a change in the current flowing in the SQUID. Thus, knowing the angular displacement covered by the movement of the apparatus, it is possible to extract the absolute value of {right arrow over (g.)}
In order to obtain the variation of {right arrow over (g)}(t) with time, one simply lets the current flow in the SQUID. If {right arrow over (g)}(t) changes with time, it will produce a change in the phase shift and a change in the current flowing in the SQUID.
In order to reduce the inductance of the SQUID and thus the effect of any stray magnetic field that might have penetrated the shielding, several designs are proposed to eliminate this problem In contrast to SQUIDs used as magnetometers, the effective area of the loop of the present invention can be as small as one wishes.